In general, frothing a liquid involves supplying a gas like air to the liquid and mixing the liquid with the gas, for the purpose of obtaining a mass of bubbles in the liquid, which is referred to as foam. In many cases, the liquid to be frothed is milk. Especially in the field of making coffee and coffee specialties, there is a need for a device which is capable of frothing milk in a user-friendly manner. It is a well-known option to combine a frothing process of milk with a heating process of milk, wherein air is supplied to the milk for obtaining foam, and wherein steam is supplied to the milk for increasing the temperature of the milk.
U.S. Pat. No. 4,715,274 discloses a unit for emulsifying steam and milk to prepare cappuccinos, which is suitable to be used in a coffee making machine. The unit comprises a steam delivery conduit in communication with a steam generator. The steam delivery conduit opens into a suction chamber whereto a milk intake conduit and an air intake conduit are led. The suction chamber is in communication with an emulsifying chamber provided with an outward dispensing opening at the bottom. At the top, the emulsifying chamber is closed by a cover which is force fitted and provided with an annular seal.
The emulsifying chamber has a substantially cylindrical configuration, whereinto the steam is admitted in a near-emulsified state with the milk, to create a turbulent flow region which furthers the emulsification and condensation. The milk and steam mixture is admitted along a tangent direction so as to promote blending on the basis of a spinning movement inside the emulsifying chamber. From the cover of the emulsifying chamber, a partition extends in a downward direction for preventing spinning of the emulsion with too high a dispensing velocity and for promoting blending together of the steam, milk, and air particles. The dimensional ratios between the several conduits are selected to provide full condensation of the steam as it is admixed to milk and air, while ensuring adequate heating of the milk.
A disadvantage of the known device resides in the fact that the emulsifying unit is adapted to delivering only one stream of milk froth, namely through the opening which is present at the bottom of the substantially cylindrical emulsifying chamber. If it is desired for the device to deliver two streams of milk froth, so that it is possible to make two cups of cappuccino in one go, a Y-shaped piece or a T-shaped piece should be used for splitting the single stream of milk froth. Using an additional piece adds to the effort which needs to be taken in cleaning the device, and also results in a need for additional height in the total arrangement.
Another device for frothing a liquid is known from WO 2012/029019. In the device, a chamber which is arranged for allowing a frothing process to take place in an environment with excess frothing gas comprises a substantially round outlet tube provided at the bottom of the chamber. The outlet tube serves for letting out excess frothing gas besides liquid in a frothed state. The chamber is typically capable of delivering only one stream of frothed liquid, wherein it is not possible to use a Y-shaped piece or a T-shaped piece as mentioned in the foregoing for splitting the stream into two if so desired, due to the presence of the excess gas and the need to let out the excess gas from the chamber. The most straightforward way to realize outlet of frothed liquid at two positions seems to be applying a simple gutter-divider operated by gravity.